Abstract: BACKGROUND:Poor fetal growth rate, as indicated by lower birth weight, has been shown to be associated with lower respiratory function in childhood; however findings in adult life remain inconsistent. A birth cohort provides the opportunity to study the association between birthweight and adult respiratory function. METHODS:The present study data are from a longitudinal birth cohort, Mater-University of Queensland Study of Pregnancy. Prospective data were available from 2368 young adults who performed standard spirometry when 21 years old. Pregnancy and birth related variables collected were: birthweight, placental weight, parental height, maternal educational status, smoking history in pregnancy, history of alcohol, tea and coffee consumption during pregnancy. The impact of birthweight on adult lung function was assessed using univariate and multivariate analyses. RESULTS:For every 100gm increase in birthweight forced vital capacity (FVC)[CI] at 21 years increased by 24mls[15-32] in males and 20mls[13-27] in females and the increase in forced expiratory volume in 1s (FEV(1))[CI] was 22 mls[15-30] and 16 mls[11-22], respectively. These associations remain adjusting for lifestyle factors during pregnancy, current smoking and parental height. However, further adjustment for adult height reduces the strength of association and remains significant for FEV(1): 8mls[1-14] in males and 5mls[1-10] in females, but not for FVC: 7mls[-1-14] in males and 5mls[-1-11] in females. CONCLUSION:Our longitudinal cohort study provides evidence of robust links between birthweight and adult lung function at the age of 21 years. Varying estimates of the effect size in the literature may be related to the age at assessment.